The present invention relates to a modulating device and, more particularly, to a modulating device advantageously applicable, but not limited to, a digital radio communications system of the type adopting multi-level quadrature amplitude modulation (QAM).
A recent achievement in the realm of digital communications art is a multi-level QAM system which remarkably enhances high performance data transmission, e.g., a 16 QAM system or a 64 QAM system. In this kind of modulation system, the occupied bandwidth of a modulation signal wave is severely restricted for the purpose of, among others, promoting efficient use of the limited bandwidth. With a modulating device of the kind generating a multi-level QAM wave, it has been customary to use filters which confine the QAM signal in a desired frequency band by effecting cosine roll-off spectral shaping. Such a modulating device may be constructed to produce a 64 QAM wave from two groups of three data streams, as well known in the art. This type of device has digital-to-analog (DA) converters each translating three data streams into an 8-level signal, two filters each being associated with a respective one of the DA converters for applying cosine roll-of spectral shaping to the 8-level signal, and a quadraphase modulator for processing the filtered eight-level signals by using the output of a local oscillator so as to deliver a QAM wave (64 QAM wave) having undergone cosine roll-off spectral shaping. Each of the filters is made up of a low-pass filter for producing a cosine roll-off spectral waveform, an amplitude equalizer for transforming a non-return-to-zero (NRZ) data signal into an impulse signal, and a delay equalizer for compensating for delay distortions which are inherent in the low-pass filter and amplitude equalizer. These components as a whole are required to have a characteristic which is free from delay distortions and is substantially coincident with theoretical amplitude values. However, since the characteristic required of the filter is extremely severe, the adjustment is time- and labor-consuming. It follows that the modulating device using the filters is disproportionately large scale and expensive.
In the light of the above, the low-pass type filters may be replaced with binary transversal filters (BTFs). A BTF which is a digital signal processing implementation eliminates the need for adjustments and can be reduced in circuit scale by large-scale integration. However, BTFs each having only one-bit input cannot implement modulating devices except for a 4 PSK modulating device, i.e., applying BTFs to a 16 QAM or higher QAM modulating device is impracticable.